1. Field of the Invention
This invention relates to a laser-diode-pumped solid-state laser, and more particularly to a laser-diode-pumped solid-state laser in which output variations and noise are reduced.
2. Description of the Prior Art
As disclosed in, for example, U.S. Pat. No. 4,656,635, a laser-diode-pumped solid-state laser is well known, wherein a solid-state laser crystal doped with rare earth elements such as Neodymium is pumped with a laser diode (a semiconductor laser). In this laser-diode-pumped solid-state laser, an optical output thereof is detected by a photodetector, and a drive current of the laser diode is subjected to a feedback control on the basis of an output obtained from that detection. Thereby, so-called APC (automatic power control) is effected for maintaining an optical output at a predetermined value. Thus, the laser-diode-pumped solid-state laser is often used while being subjected to APC.
In this type of laser-diode-pumped solid-state laser, a multi-transverse mode broad area laser is often used as a laser diode in order to obtain a high pump power. More particularly, the multi-transverse mode broad area laser, conventionally used as such a pumping source, is of the gain-guided type which forms a guide mode using a current gain.
In the conventional laser-diode-pumped solid-state laser that uses the gain guide type multi-transverse mode broad area laser as a pump source, sharp output variations (so-called kinks), resulting from variations in drive current of the laser diode itself, and minute output variations and noise, caused by returning light which is reflected from a solid-state laser crystal or the like, occur. This may lead to a considerably deteriorated linearity of drive current vs. optical output characteristics of the laser diode. In such a case, it is impossible to suppress output variations of the solid-state laser or noise even when the laser is subjected to the aforementioned APC. What is worse, this causes the APC circuit to oscillate, which in turn causes new noise. This problem is particularly noticeable when the environmental temperature around the laser-diode-pumped solid-state laser varies.